Pebble heat exchange chambers



Sept. 1l, 1956 H. J. HEPP 2,762,693

PEBBLE HEAT EXCHANGE CHAMBERS Filed March 14, 1955 INVENTOR. H J. HEP P United States Patent O PEBBLE HEAT EXCHANGE CHAMBERS Harold J. Hepp, Bartlesville, Okla., assign'or to Phillips Petroleum Company, a corporation of Delaware Application March 14, '1955, Serial No. 494,142

Claims. (Cl. 23-284) This invention relates to an improved pebble heat eX- change chamber. A specific aspect of the invention is concerned with an improved pebble heater reactor. Another aspect of the invention pertains to improved refractory construction for a pebble heat exchange chamber.

The invention is applicable to any fluid-solid heat transfer process which requires uniform heat transfer. Conventional pebble heater processes with which the invention is concerned utilize solid heat exchange material in the form of fluent particulate refractory elements called pebbles. These pebbles are usually substantially spherical and relatively uniform in size comprising spheres in the range of 1/s to l" in diameter, usually and preferably in the range of 1A to 1/2 in diameter. Pebbles commonly used include those made from alumina, beryllia, Stellite, Carborundum, mullite, periclase, zirconia, metals, and alloys. Care must be taken to select pebbles of suitable composition for any given process. In cracking hydrocarbons at elevated temperatures, for example, pebbles of a high refractory character and ruggedness must be utilized.

The conventional pebble heater process entails circulating a mass of pebbles downwardly through a series of chambers or zones, elevating them to a point above the upper chamber, and again allowing them to descend by gravity through the several chambers. The bottoms of the treating chambers are usually conical to aid pebble ow out of the chamber through a relatively narrow neck leading to the succeeding chamber or conduit. In hydrocarbon conversion processes pebbles are heated in an upper chamber by contact with a countercurrent stream of flue gas after which they pass into the conversion chamber where they heat the hydrocarbons being processed and supply the heat of reaction required. Since countercurrent flow is the usual practice, the lower part of the conversion chamber serves as a preheating zone while the conversion takes place in the upper region of the chamber. A third chamber is sometimes positioned below the conversion zone and utilized for cooling vthe pebbles before elevation and for heating air for combustion or feed gas for the process.

The conventional pebble heating chamber has a relatively large cross section and is tired externally from a furnace adjacent the lower side of the chamber. A relatively narrow column of pebbles enters the top of the chamber and expands to the full diameter of the chamber passing out of the bottom of the chamber in a relatively narrow passageway. Fuel gas burned in a furnace outside the heater or in the lower section thereof is passed through the conical bottom of the heating chamber into the mass of pebbles and rises through the pebble bed, passing out through an outlet in the upper portion of the chamber.

Apparatus of the so-called pebble heater type has been utilized in recent years for the purpose of heating fluids to elevated temperatures. Such apparatus is especially suited for use in temperature ranges above those at which Vthe best high temperature structural alloys fail.

Thus, such equipment may be used for superheating steam or other gases and for the pyrclysis of hydrocarbons to produce valuable products such as ethylene and acetylene, as well as for other reactions and purposes. Conventional pebble heater-type apparatus includes two refractory-lined contacting chambers disposed one above the other and connected by a refractory-lined passageway or pebble throat of relatively narrow cross section. Refractory solids of owable size and form, called pebbles, are passed continuously and contiguously through the system, flowing by gravity through the uppermost chamber, the throat, and the lowermost chamber, and are then conveyed to the top of the uppermost chamber to complete the cycle.

One disadvantage in the operation of conventional pebble reaction chambers is that it is most diicult to establish uniform flow of reactant materials in contact with uniformly heated pebbles from the pebble heater chamber. The reactant materials which are introduced into the reaction chamber are raised to conversion temperature by direct heat exchange with the hot solid heatexchange material in the reaction chamber and the resulting reaction products are removed from the upper portion of the reaction chamber. It is known that the gaseous material which flows upwardly through the gravirating bed of solid heat-exchange material within the reaction chamber tends to follow the path of least resistance. That path of least resistance is along the periphery of the gravitating solid material bed inasmuch as the bed is not as deep at its periphery as at its axis. The gaseous reactant material being converted in the reaction chamber tends to channel along the reactor wall, and thereby resulting in less conversion of this portion of the reactant compared with the reactant llowing nearer the center of the reactor. This channeling is due to the fact that at the wall a relatively unimpeded channel is formed since the space between adjacent pebbles where each is touching the wall cannot be partially blocked by another pebble, as is true in the close packing found among pebbles somewhat removed from the wall of the reactor.

In accordance with the present invention, channeling of the gaseous reactant material between the periphery of the gravitating bed of pebbles and the reactor ywall is substantially reduced, or minimized, by forming the face of the reactor refractory lining into a plurality of adjacent vertical, semicircular concave grooves, flutes, or channels, said vertical grooves preferably extending the entire 'length of the reactor wall and covering the entire circumferential surface of the refractory lining. The semicircular grooves are one pebble or slightly more in diameter, approximately one-half pebble diameter deep, and the adjacent vertical grooves are spaced at approximately one pebble diameter from center to center. The above-described construction of the refractory lining within the reactor closes the wall channels to gas flow, and thus etfects more uniform cracking or other reaction in the reactor. In actual operation, a row of pebbles occupy, or ll, substantially all of the space provided by the vertical semicircular. flutes. As stated before, the vertical semicircular flutes, grooves, or channels preferably extend the full length of the reactor; however, if desired, they may be less, but in any event, should probably not be less than one or two feet long, or the length of the vertical height of the reaction zone therein. The top end of the vertical grooves should be at the same level or just slightly above the pebble surface.

Some applications of the pebble heater require uniform heat exchange, particularly in the reaction chamber. An application in point is the cracking of hydrocarbons to lower molecular weight hydrocarbons without overcracking Vto coke. One ofthe problems involved in such a process is the avoidance of overcracking of a portion of the feed and under-cracking, or not cracking at all, another portion of the feed. lt has been recognized that non-uniform heating conditions exist in the heat-exchange chambers of a pebble heater due, at least in part, to non-uniformity of gas ow therethrough. It has been thought that gas flow through a pebble heating chamber is faster near the periphery of the pebble bed because of the shorter flow path resulting from the conical top or other non-horizontal condition of the pebble bed. n other words, as long as the top of the mass of pebbles in the conversion chamber is not parallel with the gas distributor in the lower section, the flow path for gas introduced at various points in the bottom of the bed is not uniform, being substantially shorter near the periphry of the bed (where the pebbles are introduced axially to the heat exchange chamber) or at other positions where the depth of the pebble bed is less.

However, there is another aspect of the situation which affects the uniformity of heat exchange and which has not been too apparent. The gases flowing longitudinally through the heat exchange chamber pass through the pebble bed from void space to Void space therein. These void spaces are relatively uniform (with uniform pebbles) in the interior of the bed because pebbles contact pebbles and the spaces between the spheres are substantially the same. At the periphery of the pebble bed where the pebbles contact the refractory wall of the chamber, small spheres Contact a generally cylindrical surface and form considerably larger void spaces therewith because the refractory surface does not extend in between adjacent pebbles in the manner that other pebbles do in the interior of the pebble bed.

Applicant has devised a pebble heater refractory lining and a pebble heater structure which substantially reduces the Void space between the periphery of the pebble bed, or the outermost layer thereof, and the contiguous refractoiy wall of the heater.

The principal object of the invention is to provide an improved pebble heat exchange chamber with improved fluid-flow characteristics. Another object of the invention is to reduce the void space in a pebble heater between the outermost pebbles and the refractory inner wall of the chamber. Another object is to provide an improved refractory construction for use in pebble heat exchange chambers. Other objects of the invention will become apparent from a consideration of the accompanying disclosure.

I have devised a fluted refractory lining for a pebble heat exchange chamber which substantially reduces the void spaces between the refractory wall of the heater and the adjacent pebbles. The flutes extend vertically in the inner surface of the refractory lining of the heat exchange chamber at least a substantial portion of the vertical height of the lining and in the upper section of the chamber. The flutes preferably extend the full length of the cylindrical section of the chamber. In some applications, such as in the reaction chamber, these flutes may be extended through only the reaction section of the heater, the lower preheating section where little or no reaction is occurring being in the form of a regular cylinder. The flutes are constructed of such a diameter that the pebbles substantially fit the flutes without binding, thereby greatly reducing void spaces along the wall of the reactor or at the surface of the lining.

A more complete understanding of the invention may be had by reference to the drawing of which Figure 1 is an elevational view of a conventional pebble heater arrangement; Figure 2 is a sectional elevation of a pebble heat exchange chamber constructed in accordance with the invention; Figure 3 is a partial vertical sectional view of another embodiment of the pebble heat exchange chamber; Figure 4 is a sectional View taken on the line 4-4 of Figure 3; Figure 5 is a view similar to Figure 4 showing the pebbles contiguous with the uted wall thereof;

and Figure 6 is a partial transverse section of the lower portion of the heat exchange chamber of Figure 3 showing a layer of pebbles contiguous with the smooth cylinv drical refractory thereof.

Referring to Figure l, a conventional pebble heater system comprises a heating chamber it) and a reaction chamber 12 connected by a throat or pebble passageway 13. A transfer system for circulating pebbles from the bottom of chamber l2 to the top of chamber i0 includes a downwardly sloping pebble conduit or chute 14 con` taining a pebble feeder l5, an elevator 16, and a delivery conduit or chute 18 leading into the upper section of pebble heating chamber lil. Pebbles are usually heated in chamber l by burning a suitable fuel introduced to the lower section of chamber lil via line 2t) in adrnixture with air admitted via line 22 so that hot combustion gas passes upwardly through chamber in direct heatexchange relationship with the gravitating mass of pebbles therein. Stack 24 carries off the cooled combustion gas.

The gas to be heated and/ or reacted is introduced to the bottom of chamber l2 via line 26 and passes upwardly through the chamber in direct heat-exchange relationship with the hot gravitating mass of pebbles in chamber l2. Products of the reaction or heated gases are withdrawn through line 23 for further treating such as quenching and various separation and recovery steps.

Referring t-o Figure 2, heat exchange chamber 12 comprises a metal shell provided with a refractory lining including a generally cylindrical section 32, a top annular section 33, and a bottom annular section 34. An outlet is provided in the bottom of the chamber through liner 34 and shell 3@ for egress `of pebbles into chute 14 (shown in Figure l). Conduit 26 connects with a gas distributing member 36 which introduces gas to the bed of pebbles within chamber .i2 over substantially the entire horizontal cross section thereof.

Cylindrical refractory liner 32 may be constructed of a single thickness of refractory material or it may be constructed in two or more separate vertical layers including insulating refractory layer 32a, in which case the innermost layer 32 is of super-refractory quality capable of withstanding maximum reaction temperatures of the order of 3000 F. and upwards. Liner 32 is usually constructed in sections of refractory material such as refractory bricks which are built into the unit in much the same manner as bricks are laid in general furnace construction work, In any event, the inner surface of refractory liner 32 is provided with llutes extending vertically along the surface of the liner from one end of the cylinder to the other. These flutes are preferably constructed a pebble diameter across and a pebble radius deep so that a transverse cross section of a ilute is semicircular in contour.

Referring to Figure 3, liner 32 is constructed so as to occupy only an upper section of chamber 12, such as the section in which the reaction is effected. Refractory liner 40 is positioned in the lower section of chamber 12 and is preferably thinner than the maximum thickness of refractory 32 so that the cylindrical inner surface of section 40, when exten-ded upwardly, is tangent to the flutes in section 32 (shown more clearly in Figure 4). If the inner cylindrical surface of refractory section 40 extends farther into the chamber an obstruction is formed extending around the chamber at the joint between refractory sections 32 and 40 and this obstruction offers resistance to pebble flow and consequently is undesirable.

Figure 4 shows a cross section of the structure shown in Figure 3 in which flutes 38 in section 32 are tangent to the inner surface of refractory section 4d, ridges 39 extending radially inwardly from the extension of inner cylindrical surface 41 of section 40. Ridges 39 may be slightly rounded at the time of construction but this is not essential because a short period of use will round these ridges to a practical shape for extended use in pebble heater service.

Figure 5 illustrates the manner in which pebbles gravitating through chamber 12 in the form of a compact mass ll the flutes in refractory liner 32 in the upper section of the chamber of Figure 3 `so as to minimize the effect of void spaces between the pebbles and the refractory wall which are shown as 46 in Figure 6 wherein the pebbles contact a smooth cylindrical refractory wall as they do in contact with refractory liner 40 in Figure 3.

While the description of the construction shown in the drawing has been limited to generally cylindrical structures, the invention is not so limited. It is feasible to utilize flutes in the inner wall of any pebble heat-exchange chamber' to obtain improved gas and pebble flow characteristics where the chamber is generally not circular in transverse cross section. The invention is also applicable to pebble heat-exchange chambers which are not ventical walled but which are in the form -of a truncated cone or prism flaring either upwardly or downwardly. It is also feasible to utilize the invention in heat-exchange chambers having non-axial pebble conduits.

While the preceding discussion has been concerned chiefly with pebble heater reaction chambers, it is to be understood that the flutes of the invention are also applicable to pebble heating chambers in which the pebbles are heated by direct heat exchange with a hot gas. In this application more uniform heating of the pebbles is obtained.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing 11nnecessary limitations on the invention.

I claim:

l. A refractory liner for a pebble heating chamber comprising a thick Wall of solid refractory material having an inner surface traversed by closely spaced utes of arcuate cross section which are upright when said liner is in position in said chamber.

2. The refractory liner of claim l wherein said flutes have a transverse cross section substantially a semicircle 1A; to l in diameter and same are spaced from center line to center line a distance approximately the diameter of said semicircle.

3. A pebble heating chamber comprising an upright metal shell lined with refractory material the inner surface of which is fiuted vertically at spaced intervals, the flutes being arcuate in cross section; means for gravitating pebbles through said chamber; and means for passing a fluid longitudinally through said chamber.

4. The pebble heating chamber of claim 3 wherein the flutes have a transverse cross section approximating a semicircle and the distance between the center lines of adjacent flutes is approximately the diameter of said semicircle.

5. The liner of claim l in the forni of a hollow cylinder open at both ends.

6. In combination a pair of cylindrical refractory liners adapted to be positioned one above the other end to end, the upper of said liners having closely spaced flutes of arcuate cross section on its inner surface and the inner surface of the lower liner being a regular cylinder.

7. The combination of claim 6 wherein the diameter of the inner surface of the lower liner is at least as large as the largest diameter of the inner surface of the fluted liner.

8. The combination of claim 7 wherein the inner surface of the lower cylinder is in alignment with a cylinder tangent to the flutes.

9. A pebble heating chamber comprising an upright cylindrical metal shell having a pebble inlet and gas outlet in the upper section and a pebble outlet and gas inlet in the lower section; a refractory liner in said shell comprising a cylindrical section and top and bottom sections fitting the top and bottom, respectively, of said shell; and closely spaced upright flutes in the inner surface of at least an upper portion of said cylindrical section, said flutes having a transverse cross section approximately a semicircle 1A to l diameter and being spaced at least a diameter distance from center line to center line.

10. The chamber of claim 9 enclosing a compact bed of pebbles of a diameter not greater than the diameter of said flutes.

Orwick et al. Jan. 20, 1931 Weber Oct. 21, 1952 

9. A PEBBLE HEATING CHAMBER COMPRISING AN UPRIGHT CYLINDRICAL METAL SHELL HAVING A PEBBLE INLET AND GAS OUTLET IN THE UPPER SECTION AND A PEBBLE OUTLET AND GAS INLET IN THE LOWER SECTION; A REFRACTORY LINER IN SAID SHELL COMPRISING A CYLINDRICAL SECTION AND TOP AND BOTTOM SECTIONS FITTING THE TOP AND BOTTOM, RESPECTIVELY, OF SAID SHELL; AND CLOSELY SPACED UPRIGHT FLUTES IN THE INNER SURFACE OF AT LEAST AN UPPER PORTION OF SAID CYLINDRICAL SECTION, SAID FLUTES HAVING A TRANSVERSE CROSS SECTION APPROXIMATELY A SEMICIRCLE 1/4" TO 1" DIAMETER AND BEING SPACED AT LEAST A DIAMETER DISTANCE FROM CENTER LINE TO CENTER LINE. 